Lipolytic enzymes play an important role in lipoprotein metabolism which, in turn, appears to play a crucial role in the development of atherosclerosis. In studying the factors which influence lipoprotein levels and metabolism, it is frequently desirable to measure body lipolytic activity concurrently with other lipoprotein parameters. The two most important lipolytic enzymes are lipoprotein lipase (LPL) and hepatic endothelial lipase (HL), each of which is believed to determine in part the levels of the triglyceride-rich lipoproteins and high-density lipoproteins (HDL). Both of these enzymes normally do not circulate in the plasma, but are bound to the endothelial cells which line the blood vessels of the body. In order to study the activity of these enzymes in human subjects, it is necessary either to biopsy the appropriate organ or to release the enzymes from the vessel wall into the circulation and then sample the blood. Heparin is chemically similar to the portion of these enzymes that bind to the vessel wall. When injected briefly, heparin then competes for that binding and releases most of LPL and HL into the blood. An appropriately timed blood sample can then be assayed in vitro for lipolytic activity of each of the enzymes by relatively simple techniques.